Screw for fastening metal or plastic sections or plates onto a base

ABSTRACT

In a screw for fastening metal sections ( 10 ) to a substructure ( 11 ), a shank ( 2 ) is provided with a threaded portion ( 5 ) and a drive portion ( 3 ) for applying a driving device. The portion of the shank ( 2 ) adjacent to the drive portion ( 3 ) flares conically toward the drive portion ( 3 ). The thread ( 5 ) engages in the substructure ( 11 ), whereby the conical portion ( 6 ) is pulled into contact with the section ( 10 ) and presses it against the substructure ( 11 ). By virtue of appropriate friction, the torque is increased to an appropriate level, and so torque-dependent stopping of the driving action can be achieved.

The invention relates to a screw for fastening metal and/or plasticsections or plates to a substructure, comprising a shank with a threadedportion and a drive portion for applying a driving tool.

The very act of fastening sections or plates to a substructure isproblematic in that such parts must achieve a condition in which theypress firmly against the substructure when fastened. These negativeeffects are intensified when relatively stiff sections or plates becomefurther distorted under certain circumstances or exhibit a longitudinaltwist. In this case it is usually not possible to work with a depthlimiter for the driving device. Even if torque clutches are used, it isstill difficult to stop the process at the precise time, since anassociated torque increase occurs as soon as the screw head bears on thesection to be fastened, thus stopping the driving device. Furtherdriving movement is then stopped regardless of whether or not the metaland/or plastic section or plate to be fastened continues to bear on thesubstructure.

The object of the present invention was therefore to provide a screw ofthe type mentioned in the introduction, in the use of which the sectionor plate to be fastened bears snugly on the substructure in the finallyset condition and torque-dependent stopping of the driving device ispossible.

According to the invention, this is achieved by the fact that theportion of the shank adjacent to the drive portion flares toward thedrive portion conically, in the manner of a stepped cone or following acurve as viewed in cross section, and that the diameter of the end ofthe flaring portion of the shank close to the drive portion is largerthan the diameter of the borehole receiving the screw in the section tobe fastened and thus is also larger than the core diameter of thethreaded portion of the screw.

By virtue of these features according to the invention, the metal and/orplastic section to be fastened is subjected to initial tension directedtoward the substructure and thus is pressed thereagainst as soon as thescrew has been driven in by one or more rotations. Because of theassociated friction in the borehole and of the flaring shape, theflaring portion of the shank exerts a force in the screwing direction onthe section to be fastened.

It then becomes a simple matter to adjust an appropriate torque clutchsuch that it disengages only at appropriately high torque. By that time,however, the section to be fastened has already been pressed snuglyagainst the substructure by the action of the flaring portion, and soall that remains is for the driving tool to be stopped as soon as theappropriate torque is reached. The torque is achieved by the highfriction of the flaring portion against the wall of the borehole. Thusrapid, torque-dependent stopping is possible, or further driving will besafely prevented.

The action of the flaring portion on the borehole wall also producesextremely good locking against reverse rotation. The surface of theflaring portion is for practical purposes disposed in a snug-fittingseat in the borehole wall. Furthermore, in the particular case ofsections that have become distorted or twisted in the process, saidsections have a tendency to return to their initial condition. Therebyan additional resilient load is always exerted on the screw,intensifying the locking effect against reverse rotation, especially inview of the action of the conical portion.

It is intuitively obvious that the beginning of the flaring portion mustextend from the shank or from an unthreaded portion of the shank,because this flaring portion, while following the shank, must beinserted easily into the borehole in the section to be fastened.Thereafter, however, it is necessary that the flaring portion engage asrapidly and effectively as possible with the borehole wall, so thatthereby it can apply the axial initial tension optimally. In this waythe tightening torque and therefore the axial initial tension areexerted on the section to be fastened as soon as the screw has beendriven in a relatively short distance, before the process of driving thescrew is fully accomplished.

Another advantage of the flaring portion is that optimum fastening to asubstructure is possible even if said substructure comprises a thinmetal sheet, other thin materials or a material of low strength, such asgas-formed concrete or even foamed material. Since the flaring portionfor practical purposes represents the agency which initiates anassociated torque increase, stripping of the thread in the substructurecannot occur.

In another proposal according to the invention, the flaring portion ofthe shank extends directly to the drive portion or to the underside ofthe drive portion formed as the screw head. This ensures that theborehole will be steadily widened until the screw is finally set.Thereby there is also achieved a steadily increasing torque, which canbe sensed by a driving tool with an adjustable stopping reaction or elseprevents further driving of the screw in some other way.

In this regard it is also important to note that torque-activatedstopping must take place with great precision, since the engagement ofthe thread in the substructure can be damaged under certaincircumstances, thus negating the fastening effect, if driving is stoppedtoo late.

It is also advantageous if the diameter of the end of the flaringportion of the shank close to the drive portion is larger than theoutside diameter of the threaded portion on the shank. It is immaterialfor the optimal effect of the present invention whether there is presentin the section or plate to be fastened a prebored borehole or else athreaded bore, in which the screw is driven. An appropriate axiallydirected initial tension is always produced by the flaring portion inthe threaded borehole of the section to be fastened or else in aborehole which is larger than the outside diameter of the thread. Ifthread turns are present, they will be compressed against or pressedflat by the flaring portion, and thereby the friction between theflaring portion and the borehole wall is smaller at least in the firsttwo rotations than if the borehole wall had been smooth. Under theseconditions, therefore, it is ensured that the sections or plates to befastened will be drawn together appropriately with the substructurebefore the driving tool is stopped or further driving of the screw isinterrupted.

An advantageous embodiment is obtained when the drive portion comprisestwo regions disposed successively in axial direction with differentstructures for a tool drive. This therefore also permits tightening witha larger or smaller wrench size as needed and thus with larger orsmaller torque transmission.

In this connection, a further advantageous embodiment is achieved whenone region of the drive portion is designed for engagement of thedriving tool and the other region is designed for application of a toolfor reverse rotation of the screw if necessary, wherein a predeterminedbreaking point acting as a stripping safeguard is formed between the tworegions of the drive portion. This creates the possibility that thescrew can be driven in with the one region of the drive portion and,after the two parts to be joined to one another have been drawnappropriately together and the torque has been increased appropriatelyby engagement of the flaring portion of the shank, the torque isincreased so much that this first region of the drive portion breaksoff. In this way, further driving of the screw is suddenly prevented,and so stripping in the substructure, especially when it has smallthickness and/or low strength, is prevented. Nevertheless, if it becomesnecessary to loosen the screwed joint, an additional region of the driveportion is still present on which another tool can be applied.

In one advantageous embodiment, the end of the shank facing away fromthe drive portion is constructed as the boring part. Such an embodimentis advantageous in particular when the two structural members to bejoined to one another do not have large thickness or when acorresponding borehole is already present in the structural member to befastened to the substructure. In this case only the borehole in thesubstructure has still to be made by the screw itself.

In the very case in which two structural members of small thickness areto be joined together, it is also possible for the end of the shankfacing away from the drive portion to be constructed as the penetrationpoint which self-taps the borehole. Such a structural geometry isexpedient in particular when, for example, structural members ofaluminum or of plastic are to be joined to one another.

If relatively rapid setting of the screw is necessary, it isadvantageous for the threaded region of the shank to be ofdouble-threaded construction. By the arrangement of the flaring portion,and specifically in connection with the predetermined breaking point inthe region of the drive portion, an optimal embodiment has been createdeven for the case of relatively rapid axial feed of the screw.

To avoid an excessively high torque already as the thread is being cutin the borehole, it is proposed that the threaded region of the shank beformed with a self-cutting thread. By means of such an embodiment athread is cut with relatively low torque, and so the danger of prematurestopping or premature break of the predetermined breaking point isprevented. In this connection it is particularly expedient for thethreaded region of the shank to be constructed in the manner of anorbiform curve, for example with trilobular shape. It has been foundthat such a design of a thread is advantageous in the very case ofinsertion in a substructure of small thickness and/or low strength.

The torque specified for stopping the driving tool or for breaking ofthe predetermined breaking point can be influenced by choice of aspecial fastening for metal and/or plastic sections or plates to asubstructure and by choice of the material of the substructure. Thus itis conceivable that the flaring portion could be trumpet-shaped. Itwould also be possible, however, for the flaring portion to bebell-shaped.

The simplest geometry in this connection is obtained when the flaringportion has the shape of a truncated cone. In this case the penetrationdepth of this portion can be predetermined by the choice of cone angleand, moreover, smaller or larger mutual friction areas between theborehole or threaded hole and the surface of the truncated-cone portionare obtained depending on cone angle.

A further advantage of the invention is that the drive portion for adriving device is designed directly as an internal drive in the flaringportion of the shank. It would therefore also be conceivable to providethe drive portion not in the form of an otherwise standard screw head,but to allow the screw to end precisely with this flaring portion.

A further possibility for influencing the friction between the flaringportion and the borehole surface cooperating therewith exists when theflaring portion is provided on its surface with projections and/ordepressions, with ribs, grooves, knurlings or the like. Thereby thefriction can be substantially increased or changed without reducing theability of the section or plate to be fastened and the substructure tobe drawn together. This additional design of the surface of the flaringportion also provides an additional improvement with respect to lockingagainst reverse rotation, especially at fastening points which areexposed to vibrations or constant load changes.

Further features and special advantages according to the invention willbe explained in more detail in the following description with referenceto the drawings, wherein:

FIG. 1 shows an elevation of an embodiment of the screw according to theinvention;

FIG. 2 shows an example of insertion of such screws in partly cutawayview;

FIG. 3 shows and enlarged view of the detail marked X in FIG. 2;

FIG. 4 shows an oblique view of an application situation for thefastening of sections according to FIG. 2;

FIG. 5 shows a further embodiment of the screw and

FIG. 6 shows a cross section through the line VI—VI in FIG. 5;

FIG. 7 to FIG. 11 show further alternative embodiments of the screwaccording to the invention;

FIG. 12 to FIG. 16 show two alternative embodiments for a screw thatcooperates with dish-like additional parts.

The screw 1 according to the invention comprises a shank 2 and a driveportion 3 formed, for example, as a screw head. Shank 2 is provided atone of its ends with a boring portion 4 and is also provided with athreaded portion 5. Portion 6 of shank 2 adjacent to drive portion 3 isflared conically toward screw head 3.

Hereinafter portion 6 will be referred to exclusively as conicallyflaring, although the geometry of this portion 6 can certainly bediverse. In addition to the conical geometry illustrated here, it wouldalso be possible to provide a structure comprising a stepped cone or aflared structure whose cross section follows a curve. Such embodimentswill be discussed in further detail with references to figures in thedrawing.

Conical portion 6 of shank 2 extends directly to underside 7 of driveportion 3. The diameter DK of the end of conical portion 6 close to thescrew head is larger than the diameter D2 of the borehole 14 whichreceives screw 1 in the section 10 to be fastened. Accordingly, diameterDK is naturally also larger than the boring diameter DB of boringportion 4. To achieve an appropriate optimum effect, diameter DK of theend of conical portion 6 close to the screw head is larger than theoutside diameter DG of threaded portion 5 on shank 2. Screw 1 is used tofasten sections 10 and if necessary intermediate layers 10′ to asubstructure 11, as can be seen in FIG. 3, for example.

The illustrated special fastening situation will be explained in furtherdetail by referring also to FIG. 4. Steel sheets 22 with trapezoidalcorrugations are laid on a beam 21 and firmly joined thereto. Afterplacement of a moisture barrier 23, sections 10 are fastened at suitablespacings to steel sheet 22 with trapezoidal corrugations, if necessarywith inclusion of intermediate layers 10′. These steel sheets withtrapezoidal corrugations have small thickness, and so precisely in sucha fastening situation it is very important not only that section 10 befirmly pressed against and held firmly on the substructure, but alsothat the screw not be stripped while being set, so that the thread canremain firmly anchored in the very thin structure of steel sheet 22 withtrapezoidal corrugations. After sections 10 have been mounted, thermalinsulation 24 is placed and finally the standing-seam roof of metal orplastic is placed thereon. Joining of standing-seam plates 25 with thesubstructure formed by steel sheet 22 with trapezoidal corrugations isachieved via sections 10, and so these are exposed to a particular loadprecisely in the case of suction forces developed by wind action.

In such a type of fastening, an important feature is that exact settingdepth of screw 1 is not required, as can be inferred from FIG. 2 and 3.Therein further driving of screw 1 is interrupted before underside 7 ofdrive portion 3 bears on upper side 26 of section 10. Such aconstruction is advantageous precisely in this application, becausethermal insulation 24 is laid directly thereon, and so no problems ofany kind arise with regard to the part of screw 1 protruding beyondupper side 26 of section 10. In the embodiment of screw 1 according toFIG. 1, 2 and 3, flaring portion 6 of shank 2 extends directly to driveportion 3 and thus to underside 7 of drive portion 3 designed as thescrew head. In another conceivable embodiment, conical portion 6 doesnot extend directly to underside 7 of drive portion 3 designed as thescrew head, but instead a short cylindrical portion, for example, whichmerges into underside 7 of drive portion 3 designed as the screw head,adjoins this conical portion 6.

A particular embodiment of drive portion 3 can also be seen from thefigures of the drawing. Drive portion 3 comprises two regions 27 and 28disposed successively in axial direction and provided with differentstructures for a tool drive. Therein region 27 of drive portion 3 isdesigned for engagement of the driving device. The other region 28,which in the illustrated practical example has a larger outsideperimeter than region 27, serves for application of a tool for reverserotation of screw 1 if necessary, if the joint had to be loosened onceagain. A predetermined breaking point 29 is provided between the tworegions 27 and 28. At least an appropriate notch effect is present hereat the transition from a thinner to a thicker material, in order therebyto achieve protection against stripping. Once conical portion 6 startsto bear against the wall of borehole 14, and once an appropriately hightorque is generated because the structural members to be joined arepressing against one another and thus conical portion 6 is exertingpressure, region 27 of drive portion 3 breaks off, and so furtherdriving of screw 1 is suddenly prevented. Thus stripping of the threadcannot occur in substructure 11, which usually comprises material ofsmall thickness or low strength.

The structural geometry of the two regions 27 and 28 and ofpredetermined breaking point 29 disposed therebetween naturally can beachieved in various ways. It would be entirely conceivable for region 28of the drive portion to be used for driving the screw and to beseparated from adjacent portion 6 by a predetermined breaking point. Insuch a case, it would also be conceivable to provide, instead ofprotruding region 27, a corresponding region as an internal drive, whichprojects into conical portion 6. Thus an external drive would be usedfor driving and an internal drive for reverse rotation. In addition, itwould be conceivable in this situation for the uppermost region ofconical portion 6 to be provided with an appropriate wrench drive, or inother words to have a region 28, which nevertheless does not projectbeyond the circumference of conical portion 6. In this connection,therefore, a plurality of structural alternatives is possible.

In several alternative embodiments according to the drawings, the end ofshank 2 facing away from drive portion 3 is constructed as boring part4. Such a geometry is suitable in particular when relatively thin partsare to be joined together or when the part to be fastened is alreadyprebored. A relatively short boring part will then suffice, since theboring process must be ended before the threaded region engages.

In the very case of the embodiment according to FIG. 11, it is clearthat screw 1 can also be constructed with a penetration tip 30 whichitself taps the borehole. This embodiment is chosen when both section 10and substructure 11 have relatively small thickness and advantageouslycomprise aluminum or plastic.

Threaded region 5 of shank 2 can have one or more threads. In the caseof multiple threads, it is advantageous to provide a double-threadedconstruction, which is particularly appropriate when relatively rapidsetting is desired.

Advantageously the threaded region of the shank is constructed with aself-cutting thread. Thereby the torque is not already too high duringcutting of the thread, and so breaking of the predetermined breakingpoint or torque-dependent stopping of the driving device in response toa large torque increase does not take place until conical section 6 hasjust been inserted. Specifically as regards the design of the thread, itis advantageous to provide a threaded region 5 in the form of anorbiform curve, for example with trilobular shape.

As already explained, flaring portion 6 can be constructed in diverseways. According to the embodiment of FIG. 8, flaring portion 6 hasbell-shaped construction, in which the angle of insertion of portion 6is relatively shallow at the beginning and then becomes progressivelysteeper. In contrast thereto, flaring portion 6 in the embodiment ofFIG. 9 is trumpet-shaped, and so the angle of insertion of portion 6increases steadily relative to the central axis of screw 1. In this wayfurther adaptation of the torque to be applied can be additionallyachieved.

The simplest embodiment of flaring portion 6 is achieved when it has theshape of a truncated cone, as can be inferred from most of theillustrated practical examples.

In the embodiments according to FIG. 10, 11, 14 and 15, no furtherspecial region of the drive portion adjoins conical portion 6. Thereinonly a drive portion in the form of an internal drive 20 is provided.

FIG. 5 and 6 show an embodiment in which flaring portion 6 has thestructure of a truncated polygonal pyramid, whereby projections and/ordepressions are obtained which act to increase the friction as portion 6is being driven into the corresponding borehole.

The embodiment according to FIG. 7 is provided with ribs 31 or grooves,which contribute to increasing the friction and also to improving safetyagainst reverse rotation.

In the scope of the invention, it is also possible to provide, in otherways, projections and/or depressions, ribs, grooves or, for example,knurlings or the like. Furthermore, different cross-sectional forms inthe region of portion 6 are possible. Thus the portion could also benon-round in cross section, for example elliptical, or could have theform of an orbiform curve, for example with trilobular shape.

It can be seen from FIG. 12 that the screw according to the invention isalso particularly suitable for use in material of low strength. Forexample, fastenings in cellular concrete or even in foamed materials areentirely possible in this way, without the risk of stripping of thethreaded region in substructure 11 despite adequate fastening.

From the embodiment according to FIG. 13 it can be inferred that a gap32 between section 10 and substructure 11 may develop under very highstress or sufficient twisting of section 10. Even at such an inherentlyundesirable fastening point, however, it is always ensured that screwretention is nevertheless present. In the case of such loading onsection 10 in the direction of arrow 33, for example in the case of suchsuction load due to wind action, section 10 is pulled correspondinglyupward and thus becomes wedged even more relative to portion 6. Therebya further improvement of locking against reverse rotation is created.

In the embodiment according to FIG. 14, a section 10 or a correspondingplate in the form of insulation enclosed on both sides with asheet-metal liner is fastened to a substructure 11. Threaded region 5 ofscrew 1 engages in substructure 11, and so flaring portion 6 engages inthe borehole on the outside of section 10, exerting pressure andincreasing friction in the process. Thus such fastening can be achievedeven without the need for a screw head or the underside of a screw headto rest snugly against the surface of a section 10.

Another alternative embodiment is illustrated in FIG. 15 and 16. In suchan embodiment, a section 10 and a substructure 11, for example, arejoined to one another, and there is placed on section 10 a kind ofprofiled washer 35, which is held securely against rotation during theprocess of driving the screw. By the action of driving threaded portion5, section 10 and substructure 11 are forced toward one another as soonas portion 6 comes into contact with funnel-shaped inside face 36 ofwasher 35. In this case also the parts are drawn correspondinglytogether and, in addition, corresponding friction is developed byflaring portion 6, in order thereby to disengage the torque clutch fromthe driving device or to cause a region of drive portion 3 to break off.In the embodiment according to FIG. 15, only an internal drive 20 ispresent, whereas in the embodiment according to 16 a drive portion 3similar to that of the embodiment according to FIG. 1 is formed.

Drive portion 3 could be provided additionally on its underside 7 withprojections and/or depressions for very special purposes. Ribs, slots,rippled ribs, roughened areas, individual points, serrated ribs or thelike could be formed here. Such a geometry would be appropriate if thesection to be fastened or the plate to be fastened also consists of amaterial of low strength, so that the flaring portion is sometimes notfully drawn into the section to be fastened or the plate to be fastened.Under these conditions the driving device would not be stoppedpractically until the last possible instant, when the torque load wouldsuddenly become extremely high. In this case also, a torque clutchhaving a mechanical, electronic or hydraulic control device willsuffice.

By virtue of the features of the invention there is always developed anappropriate tightening torque, viewed in the axial direction of thescrew, and so the sections to be fastened or a plate to be fastened areor is always pressed securely and snugly against solid substructure 11.Moreover, precisely because of the mutual bracing resulting from thebearing action of flaring portion 6 there is achieved a large torqueincrease, which at the proper time leads to torque-dependent stopping ofthe driving tool or breaking of the fastener in the region of apredetermined breaking point. Even if no axial forces are developed bythe mutual drawing together of the structural members to be fastened, anadequately large torque increase can be achieved by appropriate pressureon the fastener, by acting, for example, on the driving device. Becauseof the large torque increase when the flaring portion engages, it isassured with certainty that stopping of the driving device will takeplace before any damage can be caused to the thread in the substructure,and so a screw according to the invention can be inserted inadvantageous manner not only in thin sheets, but also in a substructureof thin material or of material of low strength. Moreover, appropriatelocking against reverse rotation is always assured.

What is claimed is:
 1. A screw for fastening metal and/or plasticsections or plates to a substructure through a borehole (14) having adiameter (D2), said screw comprising; a shank with a threaded portionhaving a core diameter (D1) and an outside diameter (DG), a driveportion for applying a driving device, and a flaring portion (6)adjacent to the drive portion (3) that flares conically toward the driveportion (3), said flaring portion (6) having a diameter (DK) close tothe drive portion that is larger than the diameter (D2) of the borehole(14) receiving the screw (1) and is also larger than the core diameter(D1) of the threaded portion (5) of the screw (1), and said driveportion (3) further comprising a region (27) having a strippingsafeguard (29) adapted to prevent stripping of the threaded portion ofthe screw within the substructure.
 2. A screw according to claim 1,wherein the flaring portion (6) of the shank (2) extends directly to thedrive portion (3) or to the underside (7) of the drive portion (3)formed as the screw head.
 3. A screw according to claim 1, wherein thediameter (DK) of the end of the flaring portion (6) of the shank (2)close to the drive portion is larger than the outside diameter (DG) ofthe threaded portion (5) on the shank (2).
 4. A screw according to claim1, wherein the drive portion (3) further comprises a region, (28)disposed successively in the axial direction with region (27), saidregions (27, 28) being adapted for engagement with different structuresof a tool drive.
 5. A screw according to claim 4, wherein region (27) ofthe drive portion (3) is adapted for engagement with a driving tool andregion (28) is adapted for engagement with a tool for reverse rotationof the screw (1).
 6. A screw according to claim 1, wherein the end ofthe shank (2) facing away from the drive portion (3) is constructed as aboring part (4).
 7. A screw according to claim 1, wherein the end of theshank (2) facing away from the drive portion (3) is constructed as apenetration point (30) which self-taps the borehole.
 8. A screwaccording to claim 1, wherein the threaded region (5) of the shank (2)is of double-threaded construction.
 9. A screw according to claim 1,wherein the threaded region (5) of the shank (2) is formed with aself-cutting thread.
 10. A screw according to claim 8, wherein thethreaded region (5) of the shank is constructed in the manner of anorbiform curve, for example with trilobular shape.
 11. A screw accordingto claim 1, wherein the flaring portion (6) is trumpet-shaped.
 12. Ascrew according to claim 1, wherein the flaring portion (6) isbell-shaped.
 13. A screw according to claim 1, wherein the flaringportion (6) has the shape of a truncated cone.
 14. A screw according toclaim 1, wherein the drive portion (3) for a driving device is designeddirectly as an internal drive in the flaring portion (6) of the shank(2).
 15. A screw according to claim 1, wherein the flaring portion (6)is provided on its surface with projections and/or depressions, withribs (31), grooves, knurlings or the like.
 16. A screw according toclaim 5, wherein the stripping safeguard (29) is formed between theregions (27, 28) of the drive portion (3).